PURPOSE: To prevent the beam irradiation work from being interrupted by negating a bias voltage increasing signal to be output from a servo-amplifier at the moment when the primary side current is rapidly increased so as to finish the discharge between a control grid and the negative pole by single shot.

CONSTITUTION: When a discharge is generated between a control grid 4 and the negative pole 3, a beam current is increased by said discharge so that a detection signal positive from a beam current detecting resistor 15 is increased while a servo-amplifier 19 tends to make a signal for raising the bias voltage for a control element 18. Then, the primary current of a bias insulation transformer 11 increases at the same time so that said increasing signal negative is supplied to the servo-amplifier 19 through a detection transformer 51, a rectifier 52, a comparator 53 differentiator (resistor 54 and capacitor 55), a waveform shaping diode 56 and an input resistor 58 while negating an upward action of the bias voltage.